Optical communications networks have recently become established as the preferred backbone for data communications because of the high bandwidth capacity of the optical fiber links. Concurrently, the internet protocol (IP) is gaining popularity for controlling the routing of packetized data across a communications network. Typically, IP packets are converted into optical form and launched into a fiber optic link towards a node of the network. When the optical signal arrives at the node, it is usually converted back into electronic form and buffered to allow the header portion of the IP packet to be extracted and decoded to determine the routing for the next hop towards the destination.
This solution suffers the disadvantage that within each node of the optical network, the IP packet must be converted from optical to electronic form and then back into optical form for transmission over the next link. Thus, while the bandwidth capacity of each fiber link may be very high, the overall bandwidth capacity of the network is restricted by the speed and reliability of the electronic components of each node, and in particular the speed with which optical signals can be converted into electronic signals and buffered. U.S. Pat. No. 5,617,233, issued to Boncek on Apr. 1, 1997, proposes a solution to this problem by providing a transparent optical node structure for use in a two-connected optical packet switching architecture. According to Boncek, each packet is separated into its respective header and payload portions. The header portion is transmitted over a link on one wavelength (or channel) and the data portion of the packet is transmitted on a second wavelength. Accordingly, at each node it is only necessary to electronically decode the header portion of the packet, which can be easily detected independently of the data portion of the packet. The decoded header portion can then be used to determine packet routing in a conventional manner, and this routing information is used to control a transparent optical switch to guide the data portion of the packet through the transparent optical switch and into a downstream link without being converted into an electronic signal and back again to an optical signal. Optical buffering of the data portion of the packet (e.g. by use of one or more fiber rings having a predetermined length) is used to ensure proper timing between the header and data portions of each packet, and may also be used to resolve contention issues and provide add/drop functionality of the node.
Known methods of buffering (or delaying) optical signals rely on the use of optical fiber rings or the like having a predetermined length. An optical signal can therefore be delayed for a period of time by launching the signal into the fiber ring. According to Boncek, such a ring is used on the input side of each node to delay the data portion of the packet for a period of time sufficient to allow the header portion to be decoded and the transparent optical switch set to provide routing of the data portion through to the appropriate output node. The delay loop on the input side of the node can also be adjusted to compensate for known differences in the propagation speed of the header and data portions of the packet through the link. A further delay ring can also be used within the transparent optical switch to resolve contention issues by enabling a data portion of a packet to be delayed by a period of time equivalent to the length of a data packet arriving on another input node, to thereby enable that other data packet to clear an output port before the first data packet arrives at that same output port.
The use of a fiber ring for delaying and buffering optical signals suffers the disadvantage that the amount of delay is governed primarily by the length of the fiber ring (the time delay is equal to the length of the ring divided by the speed of light in the fiber medium of the ring) This is generally fixed during the design and construction of the fiber ring, and cannot be varied during operation of the node. Because contention resolution requires one data packet to be delayed by a period of time equal to or greater than the length of a second data packet, assumptions must be made concerning the maximum allowable packet length during the design phase of the node. Successful operation of the network depends on the data packets never being longer than the previously defined maximum allowable length. If the defined maximum length is too large, unnecessarily long delays will be incurred as packets move through the node. Alternatively, if the defined maximum length is too small, the available band-width of the network may be unnecessarily limited.
Boncek's use of delay rings at the input of the network node imposes a further restriction that the size and content of the packet header must also be predetermined at the time of design and construction of the node, because the header portion must be fully decoded and interpreted before the delayed data portion arrives at the transparent optical switch. In light of the foregoing, it will be apparent that the transparent optical node structure of Boncek will only work in a network environment in which data packets of a uniform predefined size and format are employed.
However, in the modern network space, packetized data traffic of multiple different protocols (e.g. internet protocol, asynchronous transfer mode, etc.) is transported over a common network infrastructure. Each protocol provides its own packet (or frame) size and format standards. Additionally, some protocols (e.g. IP) are specifically designed to allow packets having widely varying lengths. New routing protocols, for example the multi protocol label switching (MPLS) protocol have been proposed to facilitate multi-protocol traffic across a common network infrastructure. Such routing protocols are commonly derived from the internet protocol, and are also specifically designed to handle data packets having widely differing format and size.
Accordingly, a system enabling full optical routing of variable length packetized data traffic across a communications network remains highly desirable.